[BOJ][C++] 2473 새 용액

https://www.acmicpc.net/problem/2473

#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
using namespace std;

int n;

int main() {
	//iostream과 stdio 버퍼 동기화시켜서 입력 빨리받아오자
	ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
	//==============================================
	cin>>n;
	vector<long long> liquids(n,0);
	for(int i = 0; i<n; i++){
		cin>>liquids[i];
	}
	sort(liquids.begin(), liquids.end());
	
	//a,b용액을 이중포문으로 찾고, 마지막 c는 lower_bound를 사용하자.
	long long a,b,c;
	long long chosen_a, chosen_b, chosen_c;
	long long min_abs = 3000000010;
	for(int i = 0; i<liquids.size()-2; i++){
		a = liquids[i];
		for(int j = i+1; j<liquids.size()-1; j++){
			b = liquids[j];
			//lower_bound
			int l,r;
			l = j+1; //low
			r = liquids.size()-1;
			int idx_c = lower_bound(liquids.begin()+l, liquids.begin()+r, -a-b)-liquids.begin();
			//l~r사이에 없을때
			if(idx_c>liquids.size()-1){
				c = liquids[liquids.size()-1];
			}
			//있으면 idx_c와 idx_c-1중 더 작은거 선택
			else if(idx_c>l){
				if(abs(a+b+liquids[idx_c])<abs(a+b+liquids[idx_c-1])){
					c = liquids[idx_c];
				}
				else{
					c = liquids[idx_c-1];
				}
			}
			//idx_c가 l일때
			else{
				c = liquids[idx_c];
			}
			if(min_abs>abs(a+b+c)){
				min_abs = abs(a+b+c);
				chosen_a = a;
				chosen_b = b;
				chosen_c = c;
			}
		}
	}
	cout<<chosen_a<<" "<<chosen_b<<" "<<chosen_c;
	return 0;
}